Electrochemical polarographic principles are employed in known oxygen sensors including an oxygen pump cell and a Nernst cell built, for example, from solid oxide electrolyte materials such as doped zirconia, and linked together through an external electrical circuit. The Nernst cell includes an air reference electrode (or a biased reference electrode) and a sensing electrode and a solid electrolyte therebetween. The pump cell includes a first and second electrode with a solid electrolyte therebetween and a gas chamber with an aperture. The first electrode of the pump cell and the sensing electrode of the Nernst cell are exposed to the gas chamber which receives a representative flow of test gas, such as engine exhaust gas. A controlled electrical potential is applied to the pump cell to pump oxygen into and out of the gas chamber to maintain the electromotive force EMF of the Nernst cell as sensed at the air reference electrode thereof at a desired potential.
To provide for sensing of the oxygen concentration in the test gas, such as by sensing oxygen flux in the gas chamber, the sensor must be maintained in a current limiting range of operation by maintaining the potential applied to the sensor within a predetermined voltage range. The current limiting range of operation is characterized by a sensor output current that is insensitive to variations in the potential applied to the pump cell. In such a range of operation, oxygen flux into or out of the gas chamber is limited by the aperture and sensor output current indicates the maximum flow that can be supported by the concentration in the test gas. If the potential is above the predetermined voltage range, additional oxygen may be stripped from gas species as H.sub.2 O and CO.sub.2, skewing the relationship between the potential applied to the sensor and sensor output current. If the potential is below the predetermined voltage range, an excess of oxygen is available and sensor output current does not indicate oxygen concentration but rather is a nonlinear function of the potential applied to the sensor.
To ensure accurate oxygen sensing in the current limiting range of operation, conventional electrochemical polarographic sensors must have stable electrocatalytic effects and must retain certain current limiting properties. If the sensor electrodes lose a portion of their electrocatalytic effect or if the current limiting means of the sensor is damaged, the linear relationship between oxygen concentration and the sensor output current can vary unpredictably, reducing sensor accuracy.
The electrocatalytic effect of commonly used platinum sensor electrodes are known to deteriorate with time, with application of high current and high voltage, and with operation at high temperature. Current limiting properties of conventional electrochemical polarographic sensors can degrade due to thermal shock, gas erosion, and impurity deposition on the sensor.
Oxygen concentration sensing is required as an input in many conventional control systems, such as automotive internal combustion engine air/fuel ratio control. Further, such information is useful in diagnostic applications, such as for diagnosing oxygen storage and release capacity of conventional catalytic treatment devices. The performance of such control and diagnostics over time requires accurate oxygen sensing. In the event the oxygen sensor cannot be maintained in the current limiting range of operation, and therefore cannot provide an output current indicating oxygen concentration, it would be desirable that such condition be diagnosed so that corrective action may be taken to preserve control and diagnostics functions dependent thereon.